Olefin polymerisation process

ABSTRACT

A process for the homopolymerisation of ethylene or the copolymerisation of ethylene and (a-olefins in a polymeristation reactor, said process carried out in the presence of a catalyst system comprising (a) a polymerisation catalyst and (b) an ionic activator is characterised in that an organometallic compound of a Group IIIB metal having at least one unit of formula: Mç0çR or Mç0çM where M is the Group IIIB metal and R is a hydrocarbyl group is added to the reactor. Preferred organometallic compounds include aluminoxanes and the process results in improved poison scavenging as well as advantages in activity profiles, catalyst activity and product characteristics. The process is particularly suitable for use with supported metallocene catalyst systems in the slurry or gas phase.

The present invention relates to polymerisation processes in particularto processes for the polymerisation of olefins and in particular toprocesses using supported metallocene catalysts. The present inventionis particularly directed to gas phase polymerisation processes.

In recent years there have been many advances in the production ofpolyolefin homopolymers and copolymers due to the introduction ofmetallocene catalysts. Metallocene catalysts offer the advantage ofgenerally a higher activity than traditional Ziegler catalysts and areusually described as catalysts which are single site in nature. Therehave been developed several different families of metallocene complexes.In earlier years catalysts based on bis (cyclopentadienyl) metalcomplexes were developed, examples of which may be found in EP 129368 orEP 206794. More recently complexes having a single or monocyclopentadienyl ring have been developed. Such complexes have beenreferred to as ‘constrained geometry’ complexes and examples of thesecomplexes may be found in EP 416815 or EP 420436. In both of thesecomplexes the metal atom eg. zirconium is in the highest oxidationstate.

Other complexes however have been developed in which the metal atom maybe in a reduced oxidation state. Examples of both the bis(cyclopentadienyl) and mono (cyclopentadienyl) complexes have beendescribed in WO 96/04290 and WO 95/00526 respectively.

The above metallocene complexes are utilised for polymerisation in thepresence of a cocatalyst or activator. Typically activators arealuminoxanes, in particular methyl aluminoxane or compounds based onboron compounds. Examples of the latter are borates such astrialkyl-substituted ammonium tetraphenyl- or tetrafluorophenyl-borates.Catalyst systems incorporating such borate activators are described inEP 561479, EP 418044 and EP 551277.

The above metallocene complexes may be used for the polymerisation ofolefins in solution, slurry or gas phase. When used in the slurry or gasphase the metallocene complex and/or the activator may be suitablysupported. Typical supports include inorganic oxides eg. silica orpolymeric supports may alternatively be used.

Examples of the preparation of supported metallocene catalysts for thepolymerisation of olefins may be found in WO 94/26793, WO 95/07939, WO96/00245, WO 96/04318, WO 97/02297 and EP 642536.

WO 98/27119 describes supported catalyst components comprising ioniccompounds comprising a cation and an anion in which the anion containsat least one substituent comprising a moiety having an active hydrogen.In this disclosure supported metallocene catalysts are exemplified inwhich the catalyst is prepared by treating the aforementioned ioniccompound with a trialkylaluminium compound followed by subsequenttreatment with the support and the metallocene.

WO99/28353 describes similar polymerisation catalyst systems whichoptionally comprise organoaluminium compounds of formula AlR_(n)X_(3-n)wherein R is typically alkyl group, X is halogen or an alkoxy group andn is 1, 2 or 3. The organoaluminium compounds are contacted with theother catalyst components as part of the catalyst preparation ratherthan separate addition to the activated catalyst. The organoaluminiumcompounds are used to prevent the active catalyst species fromchemically bonding with the support material during the catalystpreparation.

Supported metallocene catalysts have been primarily used for slurry andgas phase processes. In such processes various additives may be added tothe gas phase in order to improve the process or the characteristic ofthe resultant polymers.

For example EP 781300 describes the use of small amounts of-scavenger ingas phases processes catalysed by metallocenes to reduce the tendency offouling and sheeting. Typical scavengers described for this purposeinclude organometallic compounds such as trialkylaluminium compounds andin particular triethylaluminium. The scavengers may be used duringstart-up or added continuously in specific amounts during thepolymerisation process. The scavenger may be introduced directly orindirectly into the reactor or by any external means which allows thescavenger to enter the reactor. The metallocenes systems exemplified arethose based on bis(cyclopentadienyl) complexes for examplebis(1,3-dimethyl-n-butyl)cyclopentadienyl)zirconiumdichloride/methylaluminoxane systems.

Such trialkylaluminium compounds may however deactivate certainmetallocene catalyst systems for example metallocenes comprisingmonocyclopentadienyl complexes.

We have now surprisingly found that processes catalysed by certainpolymerisation catalysts may be improved by the addition of certainorganometallic compounds into the reactor.

Improved poison scavenging may be observed as well as advantages inactivity profiles, catalyst activity and product characteristics.

Thus according to the present invention there is provided a process forthe polymerisation of olefin monomers selected from (a) ethylene, (b)propylene (c) mixtures of ethylene and propylene and (d) mixtures of(a), (b) or (c) with one or more other alpha-olefins in a polymerisationreactor, said process being carried out in the presence of a catalystsystem comprising (a) a polymerisation catalyst and (b) an ionicactivator said process characterised in that an organometallic compoundof a Group IIIB metal having at least one unit having the formula:M—O—R or M—O—Mwhere M is the Group IIIB metal and R is a hydrocarbyl group is added tothe reactor.

Preferred organometallic compounds are those where the Group IIIB metalM is aluminium or boron and the hydrocarbyl group is a C1-C8 alkyl groupfor example methyl or isobutyl.

Preferred organometallic compounds having the unit M—O—R are metalalkoxides for example diethylaluminium ethoxide.

Preferred organometallic compounds having the unit M—O—M arealuminoxanes.

Suitable aluminoxanes include those well known in the art for examplemethyl aluminoxane (MAO). A particularly preferred aluminoxane for usein the process of the present invention is tetraisobutlyaluminoxane(TiBAO).

The organometallic compound may be added directly to the reactor or viaany suitable feed lines to the reactor.

Preferably the organometallic compound may be injected into the reactortogether with the polymerisation catalyst for example in a suitablesolvent. In this latter method the organometallic compound may suitablybe premixed with the activated polymerisation catalyst before additionto the reactor.

The organometallic compound may be added continuously to the reactor ormay added intermittently.

The preferred molar ratio of organometallic compound to polymerisationcatalyst (metal) is in the range 0.1:1 to 1000:1 and preferably 0.5:1 to500:1 and most preferably 1:1 to 100:1.

The polymerisation catalyst according to the present invention maysuitably be any polymerisation catalyst used in conjunction with anionic activator. Polymerisation catalysts typically comprise compoundshaving a transition metal.

Suitable transition metal compounds may be those based on the latetransition metals (LTM) of Group VIII for example compounds containingiron, nickel, manganese, ruthenium, cobalt or palladium metals. Examplesof such compounds are described in WO 98127124 and WO 99/12981 and maybe illustrated by [2,6-diacetylpyridinebis(2,6-diisopropylanil)FeCl₂],2,6-diacetylpyridinebis (2,4,6-trimethylanil)FeCl₂ and[2,6-diacetylpyridinebis(2,6-diisopropylanil)CoCl₂].

Other catalysts include derivatives of Group IIIA, IVA or Lanthanidemetals which are in the +2, +3 or +4 formal oxidation state. Preferredcompounds include metal complexes containing from 1 to 3 anionic orneutral ligand groups which may be cyclic or non-cyclic delocalizedπ-bonded anionic ligand groups. Examples of such π-bonded anionic ligandgroups are conjugated or non-conjugated, cyclic or non-cyclic dienylgroups, allyl groups, boratabenzene groups, phosphole and arene groups.By the term π-bonded is meant that the ligand group is bonded to themetal by a sharing of electrons from a partially delocalised π-bond.

Each atom in the delocalized π-bonded group may independently besubstituted with a radical selected from the group consisting ofhydrogen, halogen, hydrocarbyl, halohydrocarbyl, hydrocarbyl,substituted metalloid radicals wherein the metalloid is selected fromGroup IVB of the Periodic Table. Included in the term “hydrocarbyl” areC1-C20 straight, branched and cyclic alkyl radicals, C6-C20 aromaticradicals, etc. In addition two or more such radicals may together form afused ring system or they may form a metallocycle with the metal.

Examples of suitable anionic, delocalised π-bonded groups includecyclopentadienyl, indenyl, fluorenyl, tetrahydroindenyl,tetrahydrofluorenyl, octahydrofluorenyl, etc. as well as phospholes andboratabenzene groups.

Phospholes are anionic ligands that are phosphorus containing analoguesto the cyclopentadienyl groups. They are known in the art and describedin WO 98/50392.

The boratabenzenes are anionic ligands that are boron containinganalogues to benzene. They are known in the art and are described inOrganometallics, 14, 1, 471-480 (1995).

The preferred polymerisation catalyst of the present invention is abulky ligand compound also referred to as a metallocene complexcontaining at least one of the aforementioned delocalized π-bondedgroup, in particular cyclopentadienyl ligands. Such metallocenecomplexes are those based on Group IVA metals for example titanium,zirconium and hafnium.

Metallocene complexes may be represented by the general formula:LxMQnwhere L is a cyclopentadienyl ligand, M is a Group IVA metal, Q is aleaving group and x and n are dependent upon the-oxidation state of themetal.

Typically the Group IVA metal is titanium, zirconium or hafnium, x iseither 1 or 2 and typical leaving groups include halogen or hydrocarbyl.The cyclopentadienyl ligands may be substituted for example by alkyl oralkenyl groups or may comprise a fused ring system such as indenyl orfluorenyl.

Examples of suitable metallocene complexes are disclosed in EP 129368and EP 206794. Such complexes may be unbridged eg.bis(cyclopentadienyl)zirconium dichloride,bis(pentamethyl)cyclopentadienyl dichloride, or may be bridged eg.ethylene bis(indenyl) zirconium dichloride ordimethylsilyl(indenyl)zirconium dichloride.

Other suitable bis(cyclopentadienyl) metallocene complexes are thosebis(cyclopentadienyl)diene complexes described in WO 96/04290. Examplesof such complexes are bis(cyclopentadienyl)zirconium(2,3-dimethyl-1,3-butadiene) and ethylene bis(indenyl)zirconium1,4-diphenyl butadiene.

Examples of monocyclopentadienyl or substituted monocyclopentadienylcomplexes suitable for use in the present invention are described in EP416815, EP 418044, EP 420436 and EP 551277. Suitable complexes may berepresented by the general formula:CpMX_(n)wherein Cp is a single cyclopentadienyl or substituted cyclopentadienylgroup optionally covalently bonded to M through a substituent, M is aGroup VIA metal bound in a η⁵ bonding mode to the cyclopentadienyl orsubstituted cyclopentadienyl group, X each occurrence is hydride or amoiety selected from the group consisting of halo, alkyl, aryl, aryloxy,alkoxy, alkoxyalkyl, amidoalkyl, siloxyalkyl etc. having up to 20non-hydrogen atoms and neutral Lewis base ligands having up to 20non-hydrogen atoms or optionally one X together with Cp forms ametallocycle with M and n is dependent upon the valency of the metal.

Particularly preferred monocyclopentadienyl complexes have the formula:

wherein:

-   -   R′ each occurrence is independently selected from hydrogen,        hydrocarbyl, silyl, germyl, halo, cyano, and combinations        thereof, said R′ having up to 20 nonhydrogen atoms, and        optionally, two R′ groups (where R′ is not hydrogen, halo or        cyano) together form a divalent derivative thereof connected to        adjacent positions of the cyclopentadienyl ring to form a fused        ring structure;    -   X is hydride or a moiety selected from the group consisting of        halo, alkyl, aryl, aryloxy, alkoxy, alkoxyalkyl, amidoalkyl,        siloxyalkyl etc. having up to 20 non-hydrogen atoms and neutral        Lewis base ligands having up to 20 non-hydrogen atoms,    -   Y is —O—, —S—, —NR*—, —PR*—,    -   M is hafnium, titanium or zirconium,    -   Z* is SiR*₂, CR*₂, SiR*₂SIR*₂, CR*₂CR*₂, CR*═CR*, CR*₂SIR*₂, or    -   GeR*₂, wherein:

R* each occurrence is independently hydrogen, or a member selected fromhydrocarbyl, silyl, halogenated alkyl, halogenated aryl, andcombinations thereof, said

R* having up to 10 non-hydrogen atoms, and optionally, two R* groupsfrom Z* (when R* is not hydrogen), or an R* group from Z* and an R*group from Y form a ring system.,

and n is 1 or 2 depending on the valence of M.

Examples of suitable monocyclopentadienyl complexes are(tert-butylamido)dimethyl(tetramethyl-η⁵-cyclopentadienyl)silanetitaniumdichloride and(2-methoxyphenylamido)dimethyl(tetramethyl-η⁵-cyclopentadienyl)silanetitaniumdichloride.

Other suitable monocyclopentadienyl complexes are those comprisingphosphinimine ligands described in WO 99/40125, WO 00/05237, WO 00/05238and WO00/32653. A typical examples of such a complex is cyclopentadienyltitanium [tri (tertiary butyl)phosphinimine] dichloride.

Another type of polymerisation catalyst suitable for use in the presentinvention are monocyclopentadienyl complexes comprising heteroallylmoieties such as zirconium(cyclopentadienyl)tris(diethylcarbamates) asdescribed in U.S. Pat. No. 5,527,752 and WO 99161486.

Particularly preferred metallocene complexes for use in the preparationof the supported catalysts of the present invention may be representedby the general formula:

wherein:

-   -   R′ each occurrence is independently selected from hydrogen,        hydrocarbyl, silyl, germyl, halo, cyano, and combinations        thereof, said R′ having up to 20 nonhydrogen atoms, and        optionally, two R′ groups (where R′ is not hydrogen, halo or        cyano) together form a divalent derivative thereof connected to        adjacent positions of the cyclopentadienyl ring to form a fused        ring structure;    -   X is a neutral η⁴ bonded diene group having up to 30        non-hydrogen atoms, which forms a π-complex with M;    -   Y is —O—, —S—, —NR*—, —PR*—,    -   M is titanium or zirconium in the +2 formal oxidation state;    -   Z* is SiR*₂, CR*₂, SiR*₂SIR*₂, CR*₂CR*₂, CR*═CR*, CR*₂SIR*₂, or    -   GeR*₂, wherein:

R* each occurrence is independently hydrogen, or a member selected fromhydrocarbyl, silyl, halogenated alkyl, halogenated aryl, andcombinations thereof, said

R* having up to 10 non-hydrogen atoms, and optionally, two R* groupsfrom Z* (when R* is not hydrogen), or an R* group from Z* and an R*group from Y form a ring system.

Examples of suitable X groups includes-trans-η⁴-1,4-diphenyl-1,3-butadiene,s-trans-η⁴-3-methyl-1,3-pentadiene; s-trans-η⁴-2,4-hexadiene;s-trans-η⁴-1,3-pentadiene; s-trans-η⁴-1,4-ditolyl-1,3-butadiene;s-trans-η⁴-1,4-bis(trimethylsilyl)-1,3-butadiene;s-cis-η⁴-3-methyl-1,3-pentadiene; s-cis-η⁴-1,4-dibenzyl-1,3-butadiene;s-cis-η⁴-1,3-pentadiene; s-cis-η⁴-1,4-bis(trimethylsilyl)-1,3-butadiene,said s-cis diene group forming a π-complex as defined herein with themetal.

Most preferably R′ is hydrogen, methyl, ethyl, propyl, butyl, pentyl,hexyl, benzyl, or phenyl or 2 R′ groups (except hydrogen) are linkedtogether, the entire C₅R′₄ group thereby being, for example, an indenyl,tetrahydroindenyl, fluorenyl, tetrahydrofluorenyl, or octahydrofluorenylgroup.

Highly preferred Y groups are nitrogen or phosphorus containing groupscontaining a group corresponding to the formula —N(R″)— or —P(R″)—wherein R″ is C₁₋₁₀ hydrocarbyl.

Most preferred complexes are amidosilane—or amidoalkanediyl complexes.

Most preferred complexes are those wherein M is titanium.

Specific complexes suitable for use in the preparation of the supportedcatalysts of the present invention are those disclosed in WO 95/00526and are incorporated herein by reference.

A particularly preferred complex for use in the preparation of thesupported catalysts of the present invention is (t-butylamido)(tetramethyl-η⁵-cyclopentadienyl)dimethylsilanetitanium-η⁴-1,3-pentadiene.

The ionic activators of the present invention typically comprise acation and an anion and may be represented by the formula:(L*-H)⁺ _(d)(A^(d−))

wherein

L* is a neutral Lewis base

(L*-H)⁺ _(d) is a Bronsted acid

A^(d−) is a non-coordinating compatible anion having a charge of d⁻, and

d is an integer from 1 to 3.

The cation of the ionic compound may be selected from the groupconsisting of acidic cations, carbonium cations, silylium cations,oxonium cations, organometallic cations and cationic oxidizing agents.

Suitably preferred cations include trihydrocarbyl substituted ammoniumcations e.g. triethylammonium, tripropylammonium, tri(n-butyl)ammoniumand similar. Also suitable are N,N-dialkylanilinium cations such asN,N-dimethlanilinium cations.

The preferred ionic compounds used as activators are those wherein thecation of the ionic compound comprises a hydrocarbyl substitutedammonium salt and the anion comprises an aryl substituted borate.

Typical borates suitable as ionic compounds include:

-   -   triethylammonium tetraphenylborate    -   triethylammonium tetraphenylborate,    -   tripropylammonium tetraphenylborate,    -   tri(n-butyl)ammonium tetraphenylborate,    -   tri(t-butyl)ammonium tetraphenylborate,    -   N,N-dimethylanilinium tetraphenylborate,    -   N,N-diethylanilinium tetraphenylborate,    -   trimethylammonium tetrakis(pentafluorophenyl)borate,    -   triethylammonium tetrakis(pentafluorophenyl)borate,    -   tripropylammonium tetrakis(pentafluorophenyl)borate,    -   tri(n-butyl)ammonium tetrakis(pentafluorophenyl)borate,    -   N,N-dimethylanilinium tetrakis(pentafluorophenyl)borate,    -   N,N-diethylanilinium tetrakis(pentafluorophenyl)borate.

A preferred type of ionic activator suitable for use with themetallocene complexes of the present invention comprise ionic compoundscomprising a cation and an anion wherein the anion has at least onesubstituent comprising a moiety having an active hydrogen.

Suitable activators of this type are described in WO 98/27119 therelevant portions of which are incorporated herein by reference.

Examples of this type of anion include:

-   -   triphenyl(hydroxyphenyl)borate    -   tri (p-tolyl)(hydroxyphenyl)borate    -   tris (pentafluorophenyl)(hydroxyphenyl)borate    -   tris (pentafluorophenyl)(4-hydroxyphenyl)borate

Examples of suitable cations for this type of cocatalyst includetriethylammonium, triisopropylammonium, diethylmethylammonium,dibutylethylammonium and similar.

Particularly suitable are those cations having longer alkyl chains suchas dihexyldecylmethylammonium, dioctadecylmethylammonium,ditetradecylmethylammonium, bis(hydrogenated tallow alkyl)methylammoniumand similar.

Particular preferred activators of this type are alkylammoniumtris(pentafluorophenyl)4-(hydroxyphenyl)borates. A particularlypreferred activator is bis(hydrogenated tallow alkyl)methyl ammoniumtris (pentafluorophenyl) (4-hydroxyphenyl)borate.

With respect to this type of activator, a preferred compound is thereaction product of an alkylammoniumtris(pentafluorophenyl)-4-(hydroxyphenyl)borate and an organometalliccompound, for example triethylaluminium.

The preferred metal with respect to the organometallic compound isaluminium and the preferred metal for the ionic activator is boronwhereby the molar ratio of A1/B is less than 2:1. The molar ratio ofpolymerisation catalyst to ionic activator employed in the method of thepresent invention may be in the range 1:10000 to 100:1. A preferredrange is from 1:5000 to 10:1 and most preferred from 1:10 to 10:1.

It is most preferred in the present invention that the polymerisationcatalyst and ionic activator are supported.

Suitable support materials include inorganic metal oxides oralternatively polymeric supports may be used.

The most preferred support material for use with the supported catalystsaccording to the process of the present invention is silica. Suitablesilicas include Ineos ES70 and Grace-Davison 948 silicas.

The support material may be subjected to a heat treatment and/orchemical treatment to reduce the water content or the hydroxyl contentof the support material. Typically chemical dehydration agents arereactive metal hydrides, aluminium alkyls and halides. Prior to its usethe support material may be subjected to treatment at 100° C. to 1000°C. and preferably at 200 to 850° C. in an inert atmosphere under reducedpressure.

The support material may be further combined with an organometalliccompound preferably an organoaluminium compound and most preferably atrialkylaluminium compound in a dilute solvent.

The support material is pretreated with the organometallic compound at atemperature of −20° C. to 150° C. and preferably at 20° C. to 100° C.

Alternative supports for the present invention are non-porouspolystyrenes for example divinylbenzene crosslinked polystyrene.

Preferred supported polymerisation catalyst systems for use in theprocess of the present invention include those described WO 02/06357 andthe aforementioned WO 98/27119.

The process of the present invention may be directed to the solution,slurry or gas phase.

A slurry process typically uses an inert hydrocarbon diluent andtemperatures from about 0° C. up to a temperature just below thetemperature at which the resulting polymer becomes substantially solublein the inert polymerisation medium. Suitable diluents include toluene oralkanes such as hexane, propane or isobutane. Preferred temperatures arefrom about 30° C. up to about 200° C. but preferably from about 60° C.to 100° C. Loop reactors are widely used in slurry polymerisationprocesses.

The preferred process for the present invention is the gas phase.

Suitable gas phase processes of the present invention include thepolymerisation of olefins, especially for the homopolymerisation and thecopolymerisation of ethylene and α-olefins for example 1-butene,1-hexene, 4-methyl-1-pentene are well known in the art. Particularlypreferred gas phase processes are those operating in a fluidised bed.Examples of such processes are described in EP 89691 and EP 699213 thelatter being a particularly preferred process for use with the supportedcatalysts of the present invention.

Particularly preferred polymerisation processes are those comprising thepolymerisation of ethylene or the copolymerisation of ethylene andα-olefins having from 3 to 10 carbon atoms.

The present invention will now be further illustrated with reference tothe following examples:

Abbreviations TEA triethylaluminium TiBAO tetraisobutylalvminoxane IonicActivator A [N(H)Me(C₁₈₋₂₂H₃₇₋₄₅)₂][B(C₆F₅)₃(C₆H₄OH)] Complex A(C₅Me₄SiMe₂N^(t)Bu)Ti(η⁴-1,3-pentadiene

EXAMPLE 1

To a toluene solution of Ionic Activator A (1.2448 g, 90.6 μmol) wasadded hexane (0.5108 g) followed by TEA in toluene (0.0709 g, 155 μmol).The mixture was left to stand for 30 minutes and was then added dropwiseto TEA treated silica (3.0238 g, 1.4 mmol Al/gSiO2), to which hexane(0.4012 g) bad been previously added. The silica was agitated duringaddition and agitation was continued until no lumps were visible. Theimpregnated silica was allowed to stand at ambient temperature for 1hour. To a heptane solution of Complex A, (0.4708 g 91.8 μmol) was addedhexane (0.3982 g). The solution containing Complex A was added dropwiseto the Ionic Activator A impregnated silica. The silica was agitatedduring addition and agitation was continued until no lumps were visible.The impregnated silica was allowed to stand for 1 hour during which timethe silica became green. To the silica was added hexane (15 ml) and theslurry was agitated to ensure thorough mixing and left to stand atambient temperature for 30 minutes. Silica was collected by filtrationand washed with 2×15 ml of hexane. Residual hexane was removed underreduced pressure and pumping was continued for 1 hour after fluidisationceased. The dry, green free flowing powder was transferred to a storagebottle for subsequent use in polymerisation reactions.

0.1 mmol of TiBAO (1M solution in a mixture of hexane and pentane) and0.101 g of the above catalyst were premixed (<1 hour) in a catalystinjection vessel and the resulting mixture used as such in apolymerisation reaction.

EXAMPLE 2

The procedure of Example 1 was followed except that the catalyst wastreated with 0.1 mmol of diethylaluminium ethoxide instead of TiBAO andused as such in a polymerisation reaction.

EXAMPLE 3 (COMPARATIVE)

The procedure of Example 1 was followed except that the catalyst waspremixed with 0.1 mmol. of triethylaluminium instead of TiBAO and usedas such in a polymerization reaction

Polymerisation Data

The catalysts from Examples 1 and 2 were tested for ethylene 1-hexenecopolymerisation as follows:

A 2.5 l double jacketed thermostatic stainless steel autoclave waspurged with nitrogen at 70° C. for at least one hour. 200 g of PEpellets (previously dried under vacuum at 80° C. for 12 hours) or 250 gof NaCl (previously dried under vacuum at 400° C. for 12 hours) wereintroduced and the reactor was then purged three times with nitrogen (7bar to atmospheric pressure). ˜0.13 g of TEA treated silica (1.5 mmolTEA/g) was added under pressure and allowed to scavenge impurities forat least 15 minutes under agitation. The gas phase was then composed(addition of ethylene, 1-hexene and hydrogen) and a mixture of supportedcatalyst (˜0.1 g) and silica/TEA (˜0.1 g) was injected. A constantpressure of ethylene and a constant pressure ratio ofethylene/co-monomer were maintained during the run. The run wasterminated by venting the reactor and then purging the reactor 3 timeswith nitrogen. The PE powder produced during the run was then separatedfrom the PE seed bed by simple sieving. Typical conditions are asfollows:

-   Temperature: 70° C.-   Ethylene pressure: 6.5 b-   P(1-hexene)/P(ethylene): 0.004-0.008-   Hydrogen: 70-100 ml added during the gas phase composition

Average Activity Activity at 1 h Catalyst Seed Bed (g/g · h · bar) (g/g· h · bar) Example 1 NaCl 77 60 Example 2 PE pellets 44 10 Example 3NaCl 13 0

1. A process for the polymerisation of olefin monomers, comprising polymerising an olefin monomer selected from the group consisting of (a) ethylene, (b) propylene, (c) mixtures of ethylene and propylene, and (d) mixtures of (a), (b), or (c) with one or more other alpha-olefins in a polymerisation reactor in the presence of a catalyst system including (a) a polymerisation catalyst, (b) an ionic activator having a cation and an anion, wherein the anion has at least one substituent comprising a moiety having an active hydrogen, and (c) an organometallic compound of a Group IIIB metal having at least one unit of formula: M—O—R or M—O—M where M is the Group IIIB metal and R is a hydrocarbyl group, wherein an activated catalyst component including the polymerisation catalyst (a) and the ionic activator (b) is first prepared, dried and then the organometallic compound (c) is premixed with said dried activated catalyst component as a separate component before the resulting mixture is added to the reactor as the catalyst system.
 2. The process according to claim 1, wherein the Group IIIB metal is aluminium or boron.
 3. The process according to claim 1 or 2, wherein the organometallic compound is an aluminoxane.
 4. The process according to claim 3, wherein the aluminoxane is tetraisobutylaluminoxane.
 5. The process according to claim 1, wherein the organometallic compound is a metal alkoxide.
 6. The process according to claim 5, wherein the metal alkoxide is diethylaluminium ethoxide.
 7. The process according to claim 1, wherein the molar ratio of the organometallic compound to a metal content of the polymerisation catalyst is in the range of from 0.1:1 to 1000:1.
 8. The process according to claim 7, wherein the molar ratio of the organometallic compound to the metal content of the polymerisation catalyst is in the range of from 1:1 to 100:1.
 9. The process according to claim 1, wherein the polymerisation catalyst is a metallocene.
 10. The process according to claim 9, wherein the metallocene has the formula: CpMX_(n) wherein Cp is a single cyclopentadienyl or substituted cyclopentadienyl group optionally covalently bonded to M through a substituent, M is a Group VIA metal bound in a η⁵ bonding mode to the cyclopentadienyl or substituted cyclopentadienyl group, X each occurrence is hydride or a moiety selected from the group consisting of halo, alkyl, aryl, aryloxy, alkoxy, alkoxyalkyl, amidoalkyl, and siloxyalkyl having up to 20 non-hydrogen atoms and neutral Lewis base ligands having up to 20 non-hydrogen atoms or optionally one X together with Cp forms a metallocycle with M and n is dependent upon the valency of the metal.
 11. The process according to claim 9, wherein the metallocene is represented by the general formula:

wherein: R′ each occurrence is independently selected from the group consisting of hydrogen, hydrocarbyl, silyl, germyl, halo, cyano, and combinations thereof, said R′ having up to 20 non-hydrogen atoms, and optionally, two R′ groups (where R′ is not hydrogen, halo or cyano) together form a divalent derivative thereof connected to adjacent positions of the cyclopentadienyl ring to form a fused ring structure; X is a neutral η⁴ bonded diene group having up to 30 non-hydrogen atoms, which forms a Tr-complex with M; Y is —O—, —S—, —NR*—, —PR*—, M is titanium or zirconium in the +2 formal oxidation state; Z* is SiR*_(2,) CR*_(2,) SiR*₂SIR*_(2,) CR*₂CR*_(2,) CR*═CR*, CR*₂SIR*_(2,) or GeR*_(2,) wherein: R* each occurrence is independently hydrogen, or a member selected from the group consisting of hydrocarbyl, silyl, halogenated alkyl, halogenated aryl, and combinations thereof, said R* having up to 10 non-hydrogen atoms, and optionally, two R* groups from Z* (when R* is not hydrogen), or a R* group from Z* and a R* group from Y form a ring system.
 12. The process according to claim 1, wherein the ionic activator has the general formula: (L*-H)³⁰ _(d) (A^(d−)) wherein L* is a neutral Lewis base, (L*-H)⁺ _(d) is a Bronsted acid, A^(d−)is a non-coordinating compatible anion having a charge of d⁻, and d is an integer from 1 to
 3. 